Deflecting circuit for cathoderay tubes



July 13, 1948. R. D. BOADLE Er AL DEFLECTING CIRCUIT FOR CATHODETRAYTUBES Filed April 12, 1946 DEFLECT/IVG (IO/AS INVENTORS RONALD DEAN8040b? IAN AUSTER H000 ATTORNEY Patented July 13, 1948 UNITED STATESMiami? DEFLEGTING CIRCUIT FOR CATHODE- RAY TUBES Application April 12,1946, Serial No. 661,490 In Australia September 14, 1945 6 Glaims. 1

Thelpresent invention relates to deflecting circuits for cathode raytubes and more particularly to means for ensuring accurate electricalcentering of the cathode ray beam over a wide range of variation ineither sweep length or sweep repetition frequency.

The invention is primarily, but not specifically, intended for use inposition indication systems of the pulse type having means fordetermining both the direction of, and the distance to, a target orother pulse reflecting object.

Such systems comprise a rotating directive antenna that may be eitherthe transmitting antenna or the receiving antenna or both, and theyfurther comprise a suitable indicator such as a cathode ra tube havingmeans for radially deflecting the cathode ray in a direction thatchanges in synchronism with change in the antenna position.

Usually the cathode ray indicating tube is provided with a pair ofdeflecting coils which are mechanically rotated around the axis of thetube in synchronism with the rotation of the antenna. The arrangement issuch that, in the absence of deflecting potentials, the cathode ray beamis caused to impinge-upon approximately the center of the associatedfluorescent screen.

A deflecting wave, of predetermined sweep length and applied to therotating deflecting coils, contains a direct current component such thatthe cathode ray is deflected from the center of the screen towards thecircumference thereof at a known radial rate.

The cathode ray deflection or sweep is initiated simultaneously with thetransmission of each short puls of radio frequency energy from therotating antenna and the radio pulse is reflected if it strikes areflecting object. The reflected pulse is picked up by the receiver,located near the pulse transmitter, and after amplification anddemodulation is employed momentarily to vary the intensity of thecathode ray beam during its radial deflection.

As the cathode ray tube is normally blessed to beam current cut on? inthe absence of reflected signal energy, the application of a signalpulse to the control electrode serves to produce a luminous spot on thefluorescent screen.' This spot has an angular position depending uponthe direction of the reflecting object from the transmitting antenna,and has a radial position (distance from the center of the screen)depending upon the distance of said object from said transmittingantenna.

By providing on the fluorescent screen of the cathode ray tube adistance scale suitably calibrated in accordance with the rate at whichthe cathode ray beam is being radially deflected across said screen, theactual distance, corresponding tothe position at which the spotindieating the reflected pulse occurs in the radial deflection, may beread directly from the screen.

In systems of this type, in order to secure the greatest accuracy inoperation, it is essential that the cathod ray beam should commence eachradial deflection or sweep from precisely the same point on thefluorescent screen. This condition may be readily obtained, for certainrelated values of sweep length and sweep repetition frequency, byconnecting the deflecting coils in the well known so called balancedbridge deflecting circuit and by adjusting the constants of the arms ofthe bridge until all static current through the deflecting coils, in theabsence of deflecting potentials, is reduced to zero.

It has been found however, that when the length of the sweep is variedin order to enable the indicating tube to cover a greater or lesserfield of direction and distance indication, or when the sweep repetitionfrequency is varied either intentionally or unintentionally, that astatic current is caused to flow in the deflecting coils which upsetsthe electrical centering of the beam and causes the starting point ofthe deflection sweep to be displaced from the desired position which, inthe present example, is the center of the screen.

It will be obvious therefore that, if undesirable inaccuracies inindication are to be avoided, the deflecting circuit must be manuallyadjusted to the condition of electrical balance for each variation insweep length or repetition frequency.

The object of the present invention. is to overcome this disadvantage byproviding means for accurately maintaining the electrical centering ofthe cathode ray beam throughout a wide range of variation in sweeplength or sweep repetition frequency. The above objective is achieved inaccordance with the present invention by providing, in a deflectingcircuit for a cathode ray tube, means for electrically balancing thestatic current flowing through the deflecting coil pair of the absenceof deflecting potentials, and means, connected in series with saiddeflecting coil pair, for providing a path of low resistance to currentflow in one direction through said deflecting coil pair and a path ofhigh resistance to current flowing in the opposite direction throughsaid coil pair.

More specifically, the present invention consists of a deflectingcircuit for a cathode ray tube comprising an electric discharge devicehaving an anode, a cathode and at least one intervening grid electrode,said anode being connected, through an impedance unit, to a positivepotential supply source and said cathode being connected to saidpositive potential supply source through a resistive network and a pairof deflecting coils and a device of unilateral conductivity choke coilill and the'resistor II.

' the potential Figure 1 illustrates a conventional bridge circult forbalancing out the direct current component in the deflecting coil of anelectromagnetim mayibereduce'd timebase; and

Figure 2 is a modification of Figure '1 moor-H;

porating the present invention.

It will be appreciated that, in order to simplify the description, thedrawings show only that portion of the circuit of acathode raydeflecting system which is necessary for the invention.

Referring to the drawings in which like parts arein'dicated by similarreference numerals there is shown in Figure 1 a conventional circuiteman understanding of ployed in electromagnetic deflecting systems forbalancing out the direct current component in the deflecting coil. i 7'An output tube '3, which in the 'circuitlllus- 'tr'ated is of thepentode type, includes "a cathode t, a control grid 5, a screengrid G, asuppressor "grid "I, and an anode 8. The desired operating pot'entialfrom the positive terminal 9 of the potential supply source (not shown)is applied to the anode 8 of the output tube 3 through a The resistor'I-I may be omitted providing the then remaining -choke coil It has asuitable value of resistance.

' Onefree endof a pair of deflecting coils I2, which are employ-ed inany convenient manner to deflect the beam of a cathode ray tube (notshown), is connected to theanode 8 either directly or through a portionof the resistor l I. The

other free end of the deflecting coil pair I2,

which will be referred to as the lowypotential and, is connected througha conductor I3 and condensers 22 to ground I' l and thence to thecathode l of the output tube 3.

- therethrough. The deflecting coils may of course be connected eitherin series as illustrated or in parallel. I v

For the purpose of centering the electron beam of the cathode ray tube(not shown) a resistance network comprising the resistors I5, it inseries is connected between the upper end or the choke coil H] andground M. It will be noted that ground It is also common to the cathodei of the tube 3 and to the negative terminal ll of supply source.Therefore, the grounded end of the resistor I5 is connected to a pointin the circuit which is negative with respect to the anode 8 of theoutput tube'3.

The centering circuit is completed by the con- "nection of the lowpotential end of the deflecting paratively small increase incurrentinthis arm: of the bridge,' the major portion flowing in "thecircuit which includes the tube tfthe-defiecting: coils I2 andtheresistance "I5. During fly'back.

v The pointflii ."is so ""s"elected that by varying. the ratio of theimpedances of the resistor sec- I tions on each side of the tappingpoint I8, as for example by moving the tapping point or by vary-' .ing:the; value of "one resistor unit (such as the unit I5 indcated asvariable on the drawing),. the direct current flowing through thedeflecting.

coils I2, in the absence of deflecting potentials. to zero or caused toflow in either direction.

The manner in which direct current through. thedeflecting coils I2 maybe balanced out or caused to flow in either direction is as follows.v Aspreviously pointed out, the anode end of the choke coil I0 is negativewith respect to its. other end, dueto the flow 'of plate 1 currenttherethrough.

It is evident that, at somepoint on the resistance network I5, IEthe'potential is the same:

asthat of the anode end of the choke coi1 1 0..

Therefore, if the tap I8 is moved tothis p'osition,. or if the value ofthe resistanceIS is-a'djusted to' produce this condition, there will beno -"fiow of direct current through the-deflecting coils"'I-2.

If the tap I8 is moved upward fromthe 'illustrated positionf'or if thevalue of the resistancesection" I 5is"reduced, direct current will=flow' through the deflecting coils I 2 towards the anode end thereof'while, if the tap IB' is moved-down ward from the illustrated positibn,or if the value of the resistance 15 is increased, direcflcurrent willflow through the'deflecting' 'coils I2 in the opposite direction, *i.e.,' towards t'he' low potential end of said coils. Similarconditionsmay'be' obtained by-fixing the relative *values of theresistors I5, I6 and varyingthe value of that por-- tion of resistancethat is located between theanode end of the deflecting coils I2 and the,

choke coil I0.

From the foregoing description of Figure 1;.

' it will be seen that the choke coil 10" (with 'or' without additionalresistance I I);;the what and. the resistors I5, I6 form the four armsof av Wheatstone'bridge, and that the deflecting c'oils- I2, togetherwith the parallel connected damping On applying a suitable input tot'he'ter'minals:

'20, 2| of the tube 3, a saw-tooth current will the choke coil III is-made high,'there is 9, comperiods the voltage built up across thedeflecting: coils I 2' is discharged'throughthe dampingresistance I 9but the'voltagebuilt'up across the chokecoil I0 tends todischargethrough the long timeconstant path comprising the'deflecting coils I2.and the resistance I5. Thus rcversingthe'cun- 'rent in the deflectingcoils I2.

.quired position.

reverse current above referred to has not suflicient time to fall tozero between operating cycles and current will still be flowing in thedeflecting coils l2 when the next deflecting cycle cornmences.Consequently the electrical centering of the cathode ray beam isaffected and the starting point of the traces is displaced from the re-In order to correct for these conditions it is necessary to vary theimpedance ratio of the resistance sections [5, 16 or, in other words, itis necessary to unbalance the bridge. It has been found that, if adevice 23 of unilateral conductivity, such as a thermionic dioderectifier or a. metallic rectifier, is connected across the bridge asshown in Figure 2 and in series with the deflecting coils 12, so as toprovide a path of low resistance for the normal deflecting currentsthrough the deflecting coils l2 and a path of high resistance for thereverse currents produced by the voltages developed across the chokecoil 59, then the electrical balance of the circuit and hence theelectrical centering of the cathode ra beam remains constant over a widerange of variation in either sweep length or sweep repetition frequency.

With a metallic rectifier or like device 23 connected as shown in Figure2 the voltages developed across the choke coil It are discharged duringthe flyback period through two paths of short time constant; one ofthese paths being through the high internal resistance of the tube 3 andthe other path being through the high back resistance of the device 23,the deflecting coils l2 and the resistance l5. Where, however, thedevice 23 is a thermionic diode, only one discharge path is availablefor the voltage built up across the choke coil Iii. Due to theunidirectional conductivity of the thermionic diode, the path for thereverse current through the deflec ing coils l2 and resistance i5 isopen circuited and the discharge takes place through the high internalimpedance of the tube 3. The discharge is thus completed in a very shortperiod, and the centering of the cathode ray is unaffected by changes ineither sweep duration or repetition frequency.

The above invention is described as applicable to distance and directionmeasurements in which a cathode ray tube is used and in which theoathode ray beam in the tube is deflected radially from the geometric ofthe tube. The invention, however, is not solely applicable to such aninstallation since it is entirely conceivable that the same problemswould arise where a cathode ray beam is electromagnetically deflected bya similar circuit arrangement and in which the deflections occur in theusual manner across the face of the tube from one side to the other.When the rectifier 23 is included in the circuit for so deflecting thecathode ray beam, the rate of deflection as well as the amplitude ofdeflection may be changed individually or simultaneously withouteffecting the point at which the cathode ray beam begins its usefuldeflection. Accordingly, the present invention, including the use of therectifier or unilateral conducting device 23, assures that anelectromagnetically deflected cathode ray beam is returned tosubstantially the same starting point regardless of rate of deflectionor amplitude of deflection.

Having now fully described and ascertained our said invention and themanner in which it is to be performed, we declare that what we claim is:

1. In a beam deflecting circuit for cathode ray apparatus, anarrangement for accurately main- 7 ill) taining electrical centering ofa cathode ray beam throughout a wide variation in beam sweep length andsweep repetition frequency, comprising an arrangement including anelectron discharge dc vice having an anode, a cathode and at least oneintervening grid electrode, means for connecting said anode to thepositive terminal of a source of potential through an impedance unit,said cathode being connected to the negative terminal of the source ofpotential and to said positive terminal through a resistive network, anda pair of beam deflecting coils and a device of unilateral conductivityserially connected between said anode and an intermediate point on saidresistive network.

2. An arrangement according to claim 1 and wherein said electrondischarge device, impedance unit and resistive network constitute thearms of a Wheatstone bridge and said serially connected coils andunilateral conducting device constitute a diagonal of said bridge.

3. An arrangement as defined in claim 1 and wherein said intermediatepoint on the resistive network is variable in an electrical sense.

a. An arrangement according to claim 1 Wherein said unilaterallyconductive device a rectifier.

5. A circuit arrangement for deflecting a cathode ray beam in a cathoderay tube comprising a deflection tube having a cathode, a controlelectrode and an anode, means to connect the cathode to the negativeterminal of a source of potential, means including an impedance toconnect the anode to the positive terminal of the source of potential,the cathode ray beam deflection coil, a unilateral conducting device,and means for connecting the deflecting coil and unilateral conductingdevice in series between the anode of said deflection tube and anadjustable point along the source of potential thereby to assure thatthe cathode ray beam is returned to substantially the same point aftereach useful deflection stroke even though the amplitude of deflectionmay be altered.

6. A circuit arrangement for deflecting a cathode ray beam in a cathoderay tube and for assuring that the cathode ray beam is returned tosubstantially the same point after each deflection stroke regardless ofbeam deflection frequency or amplitude comprising a deflection tubehaving a cathode, a control electrode and an anode, means including animpedance to connect the anode to the positive terminal of; a source ofpotential, means to connect the cathode to the negative terminal of thesource of potential, a resistance element connected across the source ofpotential, a cathode ray beam deflection coil, a rectifier element, andmeans for connecting the deflection coil and the rectifier in seriesbetween the anode of said deflection tube and a point of adjustablepotential along the said resistance.

RONALD DEAN BOADLE. IAN ALISTAIR HOOD.

REFERENCES CHEER The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,07%,495 Vance Mar. 23, 19372,153,655 Urtel et al Apr. 11, 1939 2,182,608 Andrieu Dec. 5, 1939

